Milk lipids, which are largely triglycerides, exist in the form of globules which are surrounded by a membrane. This membrane originates from the apical plasma membrane, which envelops globules during their release from the cell. Within the cell, cytoplasmic lipid droplets appear to be the immediate precursors of milk lipid globules. Little is known about the intracellular origin and mechanism of growth of these cytoplasmic lipid droplets. The objective of this study is to gain information on the origin and growth of the cytoplasmic lipid droplet precursors of milk lipid globules. Specifically, the hypothesis that triglyceride containing, vesicle-like structures bleb from endoplasmic reticulum and fuse with each other to form cytoplasmic lipid droplets will be tested. A further prediction of the hypothesis, that these triglyceride containing vesicles fuse with cytoplasmic lipid droplets to provide triglycerides and surface coat material necessary for growth of cytoplasmic droplets, will also be tested. Experiments to determine whether proteins of the coat material on surfaces of cytoplasmic lipid droplets may be involved in interaction of droplets with the apical plasma membrane are proposed. Approaches to these questions will involve application of ultrastructural, immunologic and biochemical techniques to tissues and to isolated cell fractions. These studies focus on one aspect of the overall goal of this research program, which is to ultimately understand what controls the rate of synthesis and secretion of the constituents of milk.